Image sensor with correlated double sampling technique using switched-capacitor technology

Abstract

An output signal of an image sensor pixel, which substantially avoids fixed pattern noise contributed by the readout circuitry, is provided. The apparatus, which is used to provide an output signal that is a function of the difference between two sample signals V S1 and V S2 , includes first and second capacitor elements that are coupled together at a common terminal. A reference voltage V REF is first applied to the capacitor elements, then a first sample signal V S1 from the image sensor pixel is applied to the first capacitor element producing a charge that is transferred to the second capacitor element. A second sample signal V S2 from the image sensor pixel is then applied to the first capacitor element producing a charge that is also transferred to the first capacitor element such that V O =V S2 −V S1 +V REF .

1. A method of processing an output signal of an image sensor pixel in readout circuitry having a first capacitor element coupled to a second capacitor element, comprising the steps of:
 a. applying a reference voltage V REF  to the first and the second capacitor elements;  
 b. applying a first sample signal V S1  from the image sensor pixel to the first capacitor element placing a charge on the first capacitor element;  
 c. transferring the charge from the first capacitor element to the second capacitor element;  
 d. applying a second sample signal V S2  from the image sensor pixel to the first capacitor element placing a charge on the first capacitor element; and  
 e. transferring the charge from the second capacitor element to the first capacitor element so as to provide an output signal that is a function of the difference between the second sample signal V S2  and the first sample signal V S1 .  
 
     
     
       2. A method as claimed in  claim 1  wherein step e. comprises transferring the charge from the second capacitor element to the first capacitor element so as to provide an output signal V O  where V O =V S2 −V S1 +V REF . 
     
     
       3. A method as claimed in  claim 1  wherein V S1  is a sample voltage proportional to light intensity on the pixel and V S2  is a pixel reset voltage. 
     
     
       4. A method of processing an output signal of an image sensor pixel in readout circuitry having an operational amplifier with an input terminal, a reference terminal and an output terminal, a first capacitor element having first and second terminals with the second terminal coupled to the input terminal and a second capacitor element having first and second terminals with the second terminal coupled to the input terminal, comprising the steps of:
 a. connecting the operational amplifier reference terminal to a reference voltage V REF ;  
 b. applying the reference voltage V REF  to the first terminals of the first and the second capacitor elements;  
 c. applying a first sample signal V S1  from the image sensor pixel to the first terminal of the first capacitor element placing a charge on the first capacitor element;  
 d. transferring the charge from the first capacitor element to the second capacitor element;  
 e. applying a second sample signal V S2  from the image sensor pixel to the first terminal of the first capacitor element placing a charge on the first capacitor element; and  
 f. transferring the charge from the second capacitor element to the first capacitor element so as to provide an output signal V O  on the operational amplifier output terminal that is a function of the difference between the second sample signal V S2  and the first sample signal V S1 .  
 
     
     
       5. A method as claimed in  claim 4  wherein V O =V S2 −V S1 +V REF . 
     
     
       6. A method as claimed in  claim 4  wherein V S1  is a sample voltage proportional to light intensity on the pixel and V S2  is a pixel reset voltage. 
     
     
       7. A method of processing an output signal of an image sensor pixel in readout circuitry having an operational amplifier with an input terminal, a reference terminal connected to a first reference voltage and an output terminal, a first capacitor element having first and second terminals with the second terminal coupled to the input terminal, a second capacitor element having first and second terminals with the second terminal coupled to the input terminal, first switch means adapted to be connected between a second reference voltage and the first capacitor element first terminal, second switch means adapted to be connected between a pixel and the first capacitor element first terminal, third switch means adapted to be connected between a third reference voltage and the second capacitor element first terminal; fourth switch means connected between the operational amplifier input terminal and the output terminal; fifth switch means connected between the second capacitor element second terminal and the operational amplifier output terminal; and sixth switch means connected between the first capacitor element first terminal and the operational amplifier output terminal, comprising the steps of:
 a. opening all of the switch means;  
 b. closing the first, third and fourth switch means;  
 c. opening all of the switch means;  
 d. closing the second and fifth switch means;  
 e. opening the fifth switch means and closing the fourth switch means;  
 f. opening all of the switch means;  
 g. closing the third and sixth switch means;  
 h. reading the output voltage Vo on the operational amplifier output terminal.  
 
     
     
       8. A method as claimed in  claim 7  wherein the first, second and third reference voltages are equal to V REF . 
     
     
       9. A method as claimed in  claim 8  wherein step d. includes applying a pixel sample signal V S1  to the first capacitor element. 
     
     
       10. A method as claimed in  claim 9  wherein step e. includes applying a pixel sample signal V S2  to the first capacitor element. 
     
     
       11. A method as claimed in  claim 10  wherein V O =V S2 −V S1 +V REF . 
     
     
       12. A method as claimed in  claim 11  wherein V S1  is a sample voltage proportional to light intensity on the pixel and V S2  is a pixel reset voltage. 
     
     
       13. Readout circuitry for image sensor pixels comprising:
 first capacitor means having first and a second terminals;  
 second capacitor means having first and a second terminals;  
 amplifier means having an input terminal and an output terminal, wherein the second terminals of the first and second capacitor means are connected to the amplifier means input terminal;  
 first switch means connected to the first capacitor means first terminal;  
 second switch means connected to the second capacitor means first terminal;  
 third switch means connected between the amplifier means input terminal and output terminal;  
 fourth switch means connected between the second capacitor means second terminal and the amplifier means output terminal; and  
 fifth switch means connected between the first capacitor means first terminal and the amplifier means output terminal.  
 
     
     
       14. Readout circuitry as claimed in  claim 13  wherein the amplifier means further includes a reference terminal adapted to be connected to a reference voltage V REF . 
     
     
       15. Readout circuitry as claimed in  claim 14  wherein the first switch means comprises:
 first coupling means adapted to couple the first capacitor first terminal to a reference voltage V REF ; and  
 second coupling means adapted to couple the first capacitor first terminal to a pixel.  
 
     
     
       16. Readout circuitry as claimed in  claim 15  wherein the second switch means is adapted to couple the second capacitor means first terminal to a reference voltage V REF . 
     
     
       17. Readout circuitry as claimed in  claim 16  comprising means for controlling the first and second coupling means and the second, third, fourth and fifth switch means. 
     
     
       18. Readout circuitry as claimed in  claim 17  wherein the control means is adapted to close the second switch means, the third switch means and the first coupling means substantially simultaneously. 
     
     
       19. Readout circuitry as claimed in  claim 18  wherein the control means is adapted to close the fourth switch means and the second coupling means substantially simultaneously. 
     
     
       20. Readout circuitry as claimed in  claim 19  wherein the control means is adapted to close the third switch means and the second coupling means substantially simultaneously. 
     
     
       21. Readout circuitry as claimed in  claim 20  wherein the control means is adapted to close the second switch means and the fifth switch means substantially simultaneously. 
     
     
       22. Readout circuitry as claimed in  claim 16  wherein the first and second coupling means and the second, third, fourth and fifth switch means are transistors. 
     
     
       23. Readout circuitry as claimed in  claim 16  the first and second coupling means and the second, third, fourth and fifth switch means are CMOS transistors.